One of the fastest growing technologies over the last few years has been wireless local area network (WLAN) devices based on the Institute of Electrical and Electronic Engineers (IEEE) 802.11b standard, commonly known as “Wi-Fi.” The 802.11b standard uses the 2.4 GHz frequency of the electromagnetic spectrum and allows users to transfer data at speeds up to 11 Mbit/sec.
However, a complementary WLAN standard (IEEE 802.11a) is now available that specifies how WLAN equipment has to operate on frequencies between 5 GHz and 6 GHz (the “5 GHz band”). The 802.11a standard significantly expands the capacity of WLANs, allowing data to be exchanged at even faster rates (up to 54 Mbit/sec), but at a shorter operating range than 802.11b.
Unfortunately, the Department of Defense (DOD) operates a large number of radar systems in the 5 GHz band. The DOD has become concerned that the increasing adoption of 802.11a wireless devices will, as time goes on, cause increasing interference between the pulses that make up the signals produced by the radar systems and the pulses produced by the wireless devices. Its concern is particularly acute in today's security-conscious environment.
To accommodate both radar and 802.11a WLAN wireless devices in the same 5 GHz band, the WLAN industry developed a concept called “dynamic frequency selection,” or DFS. DFS calls for wireless devices to detect the presence of radar signals. When a radar signal is detected on a particular channel, the wireless devices are to switch automatically to another channel to avoid interfering with the radar signal. DFS would appear in theory to yield an acceptable sharing of the 5 GHz band.
However, several problems have arisen in prior art implementations of DFS. First, switching sensitivity is a serious issue. If a particular implementation of DFS provides good noise rejection, switching may occur too slowly in response to a real radar transmission, resulting in undue interference. However, if the noise rejection is reduced, switching may occur in response to noise that appears to be a real radar transmission. The resulting needless channel switch reduces the efficiency with which user data is transmitted through the WLAN, and therefore reduces its effective bandwidth.
Second, DFS can only be undertaken in a wireless device when it is receiving, not when it is transmitting, since its receiver is effectively disabled during that time. Therefore, radar signals will almost certainly go undetected when the wireless device is transmitting, increasing the risk of unwanted interference to radar systems.
Fourth, some radar systems transmit the pulses of their radar signals at a low rate. If a wireless device misses detecting even one pulse, the time interval between the two adjacent pulses may be too great for the wireless device properly to identify the radar transmissions. Again, the radar transmissions may go undetected.
Finally, the position of the 802.11a wireless device with respect to the radar may be such that radar transmissions received by the wireless device are of enhanced or diminished amplitude. The transmissions may therefore be misinterpreted as noise and ignored. In a similar vein, multipath interference may transform the pulses of the radar transmission, rendering them unrecognizable as such by the 802.11a wireless device. Again, the extent to which noise is rejected has some bearing on noninterpretation or misinterpretation of radar transmissions. In either case, the risk is that channels would not be switched quickly enough, and interference results.
While the IEEE does, in 802.11h, provide a protocol that allows radar-monitoring data to be collected from wireless devices, it sets forth no method for actually monitoring radar transmissions, nor does it specify how the radar-monitoring data should be analyzed to determine whether a radar is in operation. What is needed in the art is a comprehensive, practical DFS implementation that effectively rejects noise, but quickly and correctly identifies true radar transmissions. What is further needed in the art is a DFS implementation that is operable even when a particular wireless device is transmitting.